Define pH. What is buffer solution? Derive Henderson-Hasselbalch equation for calculating the pH of an acid buffer solution.

Explanation

The pH of a solution may be defined as negative logarithm of the activity of hydrogen ion `(a_(H^(+)))`
`pH=-log[H^(+)]`
Buffer solution: A buffer solution is the solution which can resist the change inpH on addition of small amount of acid or base. The ability of a solution of resist change in pH on addition of small amount of acid or base is called buffer solution.
Henderson-Hasselbaltch equation:
`HA+H_(2)OhArrH_(3)O^(+)+A^(-)` Partial ionisation.
`Na to Na^(+)+A^(-)` complete ionisation
`K_(a)=([H_(3)O^(+)][A^(-)])/([HA])`
Since HA is weakly dissociated, the conc. HA ion solution can be taken equal to the initial conc. of the acid. Since NaA is completly dissociated, conc. of `A^(-)` can be taken equal to the conc. of NaA (Salt)
`K_(a)=([H_(3)O^(+)]["Salt"])/(["Acid"])`
`[H_(3)O^(+)]=(K_(a)["Acid"])/(["Salt"])`
`-log[H_(3)O^(+)]=-logK_(a)-"log"(["Acid"])/(["Salt"])`
`pH=pKa+"log"(["Salt"])/("Acid")`